1. Field of the Invention
The present invention relates to heating of subsea pipelines. More particularly, the invention relates to electrically heating a pipeline that is electrically insulated from seawater.
2. Description of Related Art
Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite wells are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. Some of these pipelines extend through water that is thousands of feet deep and where temperatures of the water near the sea floor are in the range of 40xc2x0 F. The hydrocarbon fluids, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbon fluids and any water present begin to cool, phenomena occur that may significantly affect flow of the fluids through the pipelines. Some crude oils become very viscous or deposit paraffin when the temperature of the oil drops, making the oil practically not flowable. Hydrocarbon gas under pressure combines with water at reduced temperatures to form a solid material, called a xe2x80x9chydrate.xe2x80x9d Hydrates can plug pipelines and the plugs are very difficult to remove. In deep water, conventional methods of depressuring the flow line to remove a hydrate plug may not be effective. Higher pressures in the line and uneven sea floor topography require excessive time and may create more operational problems and be costly in terms of lost production.
The problem of lower temperatures in subsea pipelines has been addressed by placing thermal insulation on the lines, but the length of some pipelines makes thermal insulation alone ineffective. Increased flow rate through the lines also helps to minimize temperature loss of the fluids, but flow rate varies and is determined by other factors. Problems of heat loss from a pipeline increase late in the life of a hydrocarbon reservoir because production rates often decline at that time. Problems become particularly acute when a pipeline must be shut-in for an extended period of time. This may occur, for example, because of work on the wells or on facilities receiving fluids from the pipeline. The cost of thermal insulation alone to prevent excessive cooling of the lines becomes prohibitive under these conditions.
Heating of pipelines by bundling the lines with a separate pipeline that can be heated by circulation of hot fluids has been long practiced in the industry. Also, heating by a variety of electrical methods has been known. Most of the proposals for electrical heating of pipelines have related to pipelines on land, but in recent years industry has investigated a variety of methods for electrical heating of subsea pipelines. (xe2x80x9cDirect Impedance Heating of Deepwater Flowlines,xe2x80x9d OTC 11037, May, 1999)
Two configurations for electrical heating have been considered. In one method of electrical heating, a pipe-in-pipe subsea pipeline is provided by which a flow line for transporting well fluids is the inner pipe and it is surrounded concentrically by and electrically insulated from an electrically conductive outer pipe until the two pipes are electrically connected at one end. Voltage is applied at the opposite end and electrical current flows along the exterior surface of the inner pipe and along the interior surface of the outer pipe. This pipe-in-pipe method of heating is disclosed, for example, in commonly assigned application Ser. No. 08/625,428, filed Mar. 26, 1996, now U.S. Pat. No. 6,624,428 issued Jul. 24, 2001.
In a second configuration, a single flowline is electrically insulated and current flows along the flowline. This is called the xe2x80x9cSHIPxe2x80x9d system (Single Heated Insulated Pipe). Two SHIP systems have been considered: the fully insulated system, requiring complete electrical insulation of the flowline from the seawater, and the earthed-current system, requiring electrical connection with the seawater through anodes or other means. For both systems, current is passed through the flowline pipe.
An earthed-current system developed in Norway does not require the use of insulating joints or defect-free insulation, and hence avoids the problem of shorting by water and the effects of coating defects. (xe2x80x9cIntroduction to Direct Heating of Subsea Pipelines,xe2x80x9d overview by Statoil, Saga et al, February 1998). In that system, power is connected directly to the pipe at each end of a heated section and electrodes connected to the pipe along the pipeline are exposed to seawater. This configuration allows current to flow in both the pipe and the seawater, therefore eliminating potential drop across the insulation on the pipe, so that a defect in the pipe insulation does not result in electrical failure. Since the heated section is not electrically isolated from the rest of the pipeline by insulation joints, some means must be provided to prevent current from flowing along the pipeline to areas where it may cause corrosion damage or interfere with control systems. This is accomplished by means of a buffer zone, which is a length of pipe approximately 50 meters in length between the power connection where current enters or leaves the pipeline and adjacent structures. In that design, the buffer zone may incorporate a series choke to further impede leakage currents. This method requires that the return cable be as close to the pipe as possible, or electrical efficiency will be impractically low. This configuration is not practical for some deepwater applications and the system is considerably less energy-efficient than a fully insulated system.
A fully insulated method of electrically heating a pipeline is disclosed in U.S. Pat. No. 6,049,657. In this method, an electrically insulated coating covers a single pipeline carrying fluids from a well. An alternating current is fed to one end of the pipeline through a first insulating joint near the source of electrical current and the current is grounded to seawater at the opposite end of the pipe to be heated through a second insulating joint. The single heated insulated pipe method of electrical heating of pipelines offers many advantages, but it has been found that the method utilizing the second insulating joint has a limitation. When water is present in the pipeline, the water will settle-out when the line is shut-in and can cause the second insulating joint to become electrically shorted. Hence, there is a need for apparatus and method that do not require the second insulating joint. After the second insulating joint at the remote end of the pipeline is eliminated, special configurations for minimizing leakage of electrical current beyond the section of the pipeline to be heated are needed. A configuration for minimizing voltage required for heating is also needed, since there is a voltage limit for the electrical insulation placed on a pipeline and this limit may determine the maximum length of heated pipeline. Thus, there remains a clear need for economical apparatus and method for electrical heating of a single insulated subsea pipeline without an insulating joint on or near the seafloor.
Toward providing these and other advantages, apparatus and method are provided for enhancing the flow of fluids by heating a subsea pipeline that is electrically insulated from seawater over the segment of line that is to be heated.
In one embodiment, apparatus and method are provided for heating a segment of a subsea pipeline by applying electrical voltage through an insulating joint at the host end of the segment, where a power supply is normally located, and that is elevated so that water cannot collect in the insulating joint. The power supply may be a conventional electrical generator supplying alternating current. The electrical current flows axially along the metal walls of the pipeline to an electrical connector at the remote end of the segment. Current is then directed to a seawater electrode and returns to another seawater electrode connected to the power supply. A buffer zone, that may be electrically or thermally insulated over at least a portion of its surface, is provided in the pipeline beyond the electrical connector. The buffer zone decreases the effects of current flow outside the segment between the insulating joint and the electrical connector. This current flow can affect an adjoining structure or equipment. The buffer zone may be heated by a heater powered with the current flowing through the pipeline. Electrical chokes may be used to increase impedance to current flow along the pipeline in the buffer zone. The chokes may enclose the pipeline only, the pipeline and cable or may be made with concentric conductors to provide a more uniform magnetic field.
In another embodiment, current flow outside the heated segment to complete an electrical circuit back to the power supply is provided by electrical cables. In this embodiment, seawater electrodes are not used for the heating current along the pipeline, and may or may not be used for safe discharge of the leakage current into the seawater. Heating of the buffer zone and use of electrical chokes are also provided in this embodiment.
In yet another embodiment, no insulating joint is utilized and electrical voltage is applied to electrical connectors at the midpoint of the electrically insulated segment that is to be heated and electrical connectors are placed at each end of the segment. Return current to the power supply may flow through seawater, via seawater electrodes, or by cables. Buffer zones outside the heated segment minimize the effects of current flow to adjacent structures or equipment. Current flow in the buffer zones may be decreased by use of electrical chokes and seawater electrodes. Buffer zones may be heated by external heaters in series with current flow through the electrically insulated segment of the pipeline between the electrical connectors.